I am trying to have a set up for third harmonic generation using BBO crystal. I can have up to 200 mw conversion from a BBO at 400 nm, then putting another crystal BBO (equal) I don't see any third harmonic generation.
Am I wrong ?
I have to put also the fundamental frequency on the same crystal ?
I am not sure if I really understand your question. I assume you use a combination of 2nd harmonic generation and sum frequency generation (therefore the 2nd crystal). If this is true, you need to make the 2nd Harmonic and the residual light (the fundamental frequency) overlap in the second crystal to achieve sum frequency generation. Wether or not a BBO crystal is at all appropriate for sum-frequency generation at your wavelength I cannot tell you. Good luck!
Ok, now I understand your question in my topic and give answer again.
Crystal (BBO or other) for THG will have another angular orientation than crystal for SHG. One way is to use BBO for type 1 mixing (angle 44.3 deg), but you need additional special waveplate before this crystal to make the same polarisations of 800 nm and 400 nm beams.
Another way is to use type 2 BBO (angle 55.5 deg), without additional waveplates, but the performance of THG will be worst in comparison with type 1.
So, at first you need in another crystal to obtain THG. If you have not such crystal now, I recommend you to check also LBO specs. And, in any case, if you need to get significant THG efficiency, I recommend to discuss your task in details with experienced people first.
I am well versed in optimizing SHG & THG processes. You say you have 200 mW of power at the SHG crystal. But what is your beam format ? That is: Is your laser CW or pulsed ? What is your Repetition Rate if your laser is pulsed. If pulsed, what is your pulse width ? What is the size of the beam at your SHG crystal. All these things are important to optimizing 3d harmonic generation. What are your efficiency requirements ? Do you need >10% conversion from fundamental to third harmonic ? Or do you just need several microwatts of THG.
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